Make borrowck's notion of scopes consistent with trans's notion of scopes

Make borrowck's notion of scopes consistent with trans's notion of scopes

This eliminates an ICE in trans where the scope for a particular
borrow was a statement ID, but the code in trans that does cleanups
wasn't finding the block with that scope. As per #3860

preserve looks at a node ID to see if it's for a statement -- if it
is, it uses the enclosing scope instead when updating the map that
trans looks at later.

I added a comment noting that this is not the best fix (since it may
cause boxes to be frozen for longer than necessary) and referring
to #3511.

r=nmatsakis